功能需求（functional requirement）为一软件工程用语，功能需求定义一个软件系统或组件的功能，也是一个系统需提供的功能及服务。功能可以用一组输入、行为及输出的组合来表示。功能需求可以是计算、技术细节、数据处理或其他说明系统希望达成功能的内容。功能需求会以非功能性需求（或是品质需求）为其基础，后者会描述设计或实现时的限制条件（例如性能需求、保安性或可靠度等）。

若依照需求工程的定义，功能需求会列出系统的特定结果，而非功能性需求会列出系统的一些整体特性（如成本及可靠度），功能需求有关系统的应用架构，而非功能性需求和系统的技术架构有关。

In Software engineering and systems engineering, a functional requirement defines a function of a system or its component. A function is described as a set of inputs, the behavior, and outputs.

Functional requirements may be calculations, technical details, data manipulation and processing and other specific functionality that define what a system is supposed to accomplish. Behavioral requirements describing all the cases where the system uses the functional requirements are captured in use cases. Functional requirements are supported by non-functional requirements (also known as quality requirements), which impose constraints on the design or implementation (such as performance requirements, security, or reliability). Generally, functional requirements are expressed in the form "system must do <requirement>", while non-functional requirements are "system shall be <requirement>". The plan for implementing functional requirements is detailed in the system design. The plan for implementing non-functional requirements is detailed in the system architecture.